Conventionally, with utilizing a change in a relationship between a slip ratio of tires and an acceleration of a four-wheeled vehicle, a method for judging a decrease in tire air-pressure by obtaining the relationship between the slip ratio and the acceleration from a result of primary regression calculation is proposed (for example, refer to Japanese Unexamined Patent Publication No. 211925/2003.
However, the above method is to perform regression analysis based on the least square method. Accordingly, there is a problem that even in the case where a road surface unevenness is changed within a sampling time for performing the regression analysis, only an averaged regression line is obtained but a precise regression line corresponding to the road surface unevenness is not obtained. In the primary regression analysis used in Japanese Unexamined Patent Publication No. 211925/2003, there is a problem that calculation number of times is increased and a lot of multiplication and division which impose a heavy load on a calculator are included in the calculation.